DESCRIPTION

The cpufreq driver provides a unified kernel and user interface to CPU
frequency control drivers. It combines multiple drivers offering
different settings into a single interface of all possible levels. Users
can access this interface directly via sysctl(8) or by indicating to
/etc/rc.d/power_profile that it should switch settings when the AC line
state changes via rc.conf(5).

SYSCTLVARIABLES

These settings may be overridden by kernel drivers requesting alternate
settings. If this occurs, the original values will be restored once the
condition has passed (e.g., the system has cooled sufficiently). If a
sysctl cannot be set due to an override condition, it will return EPERM.
The frequency cannot be changed if TSC is in use as the timecounter.
This is because the timecounter system needs to use a source that has a
constant rate. The timecounter source can be changed with the
kern.timecounter.hardware sysctl. Available modes are in
kern.timecounter.choice sysctl entry.
dev.cpu.%d.freq
Current active CPU frequency in MHz.
dev.cpu.%d.freq_levels
Currently available levels for the CPU (frequency/power usage).
Values are in units of MHz and milliwatts.
dev.DEVICE.%d.freq_settings
Currently available settings for the driver (frequency/power
usage). Values are in units of MHz and milliwatts. This is
helpful for understanding which settings are offered by which
driver for debugging purposes.
debug.cpufreq.lowest
Lowest CPU frequency in MHz to offer to users. This setting is
also accessible via a tunable with the same name. This can be
used to disable very low levels that may be unusable on some
systems.
debug.cpufreq.verbose
Print verbose messages. This setting is also accessible via a
tunable with the same name.

SUPPORTEDDRIVERS

The following device drivers offer absolute frequency control via the
cpufreq interface. Usually, only one of these can be active at a time.
acpi_perf ACPI CPU performance states
est Intel Enhanced SpeedStep
ichss Intel SpeedStep for ICH
powernow AMD PowerNow! and Cool'n'Quiet for K7 and K8
smist Intel SMI-based SpeedStep for PIIX4
The following device drivers offer relative frequency control and have an
additive effect:
acpi_throttle ACPI CPU throttling
p4tcc Pentium 4 Thermal Control Circuitry

KERNELINTERFACE

Kernel components can query and set CPU frequencies through the cpufreq
kernel interface. This involves obtaining a cpufreq device, calling
cpufreq_levels() to get the currently available frequency levels,
checking the current level with cpufreq_get(), and setting a new one from
the list with cpufreq_set(). Each level may actually reference more than
one cpufreq driver but kernel components do not need to be aware of this.
The total_set element of structcf_level provides a summary of the
frequency and power for this level. Unknown or irrelevant values are set
to CPUFREQ_VAL_UNKNOWN.
The cpufreq_levels() method takes a cpufreq device and an empty array of
levels. The count value should be set to the number of levels available
and after the function completes, will be set to the actual number of
levels returned. If there are more levels than count will allow, it
should return E2BIG.
The cpufreq_get() method takes a pointer to space to store a level.
After successful completion, the output will be the current active level
and is equal to one of the levels returned by cpufreq_levels().
The cpufreq_set() method takes a pointer a level and attempts to activate
it. The priority (i.e., CPUFREQ_PRIO_KERN) tells cpufreq whether to
override previous settings while activating this level. If priority is
higher than the current active level, that level will be saved and
overridden with the new level. If a level is already saved, the new
level is set without overwriting the older saved level. If cpufreq_set()
is called with a NULL level, the saved level will be restored. If there
is no saved level, cpufreq_set() will return ENXIO. If priority is lower
than the current active level's priority, this method returns EPERM.

DRIVERINTERFACE

Kernel drivers offering hardware-specific CPU frequency control export
their individual settings through the cpufreq driver interface. This
involves implementing these methods: cpufreq_drv_settings(),
cpufreq_drv_type(), cpufreq_drv_set(), and cpufreq_drv_get().
Additionally, the driver must attach a device as a child of a CPU device
so that these methods can be called by the cpufreq framework.
The cpufreq_drv_settings() method returns an array of currently available
settings, each of type structcf_setting. The driver should set unknown
or irrelevant values to CPUFREQ_VAL_UNKNOWN. All the following elements
for each setting should be returned:
struct cf_setting {
int freq; /* CPU clock in MHz or 100ths of a percent. */
int volts; /* Voltage in mV. */
int power; /* Power consumed in mW. */
int lat; /* Transition latency in us. */
device_t dev; /* Driver providing this setting. */
};
On entry to this method, count contains the number of settings that can
be returned. On successful completion, the driver sets it to the actual
number of settings returned. If the driver offers more settings than
count will allow, it should return E2BIG.
The cpufreq_drv_type() method indicates the type of settings it offers,
either CPUFREQ_TYPE_ABSOLUTE or CPUFREQ_TYPE_RELATIVE. Additionally, the
driver may set the CPUFREQ_FLAG_INFO_ONLY flag if the settings it
provides are information for other drivers only and cannot be passed to
cpufreq_drv_set() to activate them.
The cpufreq_drv_set() method takes a driver setting and makes it active.
If the setting is invalid or not currently available, it should return
EINVAL.
The cpufreq_drv_get() method returns the currently-active driver setting.
The structcf_setting returned must be valid for passing to
cpufreq_drv_set(), including all elements being filled out correctly. If
the driver cannot infer the current setting (even by estimating it with
cpu_est_clockrate()) then it should set all elements to
CPUFREQ_VAL_UNKNOWN.

SEEALSO

AUTHORS

Nate Lawson
Bruno Ducrot contributed the powernow driver.

BUGS

The following drivers have not yet been converted to the cpufreq
interface: longrun(4).
Notification of CPU and bus frequency changes is not implemented yet.
When multiple CPUs offer frequency control, they cannot be set to
different levels and must all offer the same frequency settings.